This invention relates to a continuous gravity flow apparatus for heating and/or drying of particulate material. The invention is especially useful in connection with particulate material that, while undergoing thermal treatment, is prone to forming bonds resulting in sticking or bridging within the mass of material being treated. Such sticking or bridging will disrupt the rate of material flow thereby reducing the efficiency of the treatment operation. It is also possible that the sticking or bridging will cause non-uniformity from the standpoint of the desired treatment results.
The invention has particular application with respect to the treatment of polyester pellets in a solid phase polymerization process after initial crystallization and before introduction to a reactor. Thus, after initial crystallization, it has been found to be desirable to preheat such material before it is exposed to the reactor.
Devices currently used for a solid phase polymerization process are generally of two types, one type using indirect heat transfer through a horizontal hollow agitating rotor positioned in a jacketed trough. The Solidaire and TorusDisc equipment manufactured by Hosokawa Bepex Corporation of Minneapolis, Minn. are examples of such indirect heat transfer equipment.
The second type comprises equipment where the heat is transferred by direct contact of the particulate material with heated gas as in the case of a cross flow square shaped vertical gravity flow dryer. The Buhler Roof Dryer is an example of such equipment.